German patent DE 198 36 269 A describes a method in which the tamper frequency is controlled according to a predetermined relationship dependent on the sampled setting angle of the screed. Since only the setting angle is considered as the influencing variable, the operator must adjust further influencing variables according to his own judgement, i.e. the operator cannot directly control the laying process, but rather must himself select certain machine parameters, whereby he is supported by the closed-loop control of the tamper frequency in dependence of the setting angle. It is the operator's responsibility to select the machine parameters to be entered such that he ultimately achieves the required result.
German patent DE 40 40 029 A discloses a method in which the tamper frequency is varied according to a predetermined relationship in dependence of the laying rate. Further machine parameters must be chosen and entered by the operator so that no direct control of the laying process is possible.
European patent EP 1 179 636 A discloses a road finisher, having a screed which has a tamper in the pre-compaction system. The tamper rotational speed is remotely adjustable in a closed-loop control circuit in order to vary the frequency of the tamper strokes. The effect of a change of the tamper rotational speed on the pre-compaction result is however only slight.
A road finisher disclosed in German patent DE 200 10 498 U1 has a tamper in the screed in the pre-compaction system which can be driven through an eccentric drive with fixed amplitude, i.e. a fixed tamper stroke, and optionally a vibration device for the smoothing plate. At the back of the smoothing plate a press strip is arranged, which lies permanently on the surface of the pre-compacted covering and is hydraulically subjected to non-zero crossing force pulses to produce high compaction. The press strip is not a constituent part of the pre-compaction system.
With an automatic tamper control system as disclosed in European patent EP 1 258 564 A the tamper frequency or the number of strokes of the tamper strip per unit of distance travelled can be set or automatically adapted depending on changes in the laying speed. The tamper stroke is not changed. The tamper frequency has a relatively slight influence on the pre-compaction result.
From the European patent application with prior seniority, application number 09 014 516.0, suggests at least automatically varying the tamper stroke, taking various laying parameters into account. Here too no direct process control is possible, because the setting angle of the screed or a change of the setting angle occurring during laying is not taken into account. The setting angle of the screed would namely be a very useful indicator of the operating state or operating point of the pre-compaction system, because a setting angle that is too flat or too steep inevitably results in problems with regard to the achievable compaction, flatness and structure of the covering.
Furthermore, in practice leveling systems or even automatic leveling systems for road finishers are known which typically include closed-loop control circuits and actuate the leveling cylinder of the road finisher. At least one sensor samples a reference and supplies a measurement which is compared in the system with a target value. An operator enters the target value. The system then generates the actuating signal for the leveling cylinder. The paving thickness is controlled in this way, whereby other essential machine parameters for the paving thickness, such as the tamper stroke, tamper frequency and the laying rate, are not considered. The machine parameters must be optimally chosen and implemented by the operator. The machine parameters are not process target values, which implies that the operator cannot set that which is to be strived for, namely a certain paving thickness. The operator so to speak controls the machine to ultimately achieve the required result. A direct process control is not possible with this method.